In a gas turbine engine, various components thereof are subjected to high temperatures and high pressures so that they inevitably suffer degradation over time, and this may prevent the gas turbine engine from operating at an originally designed operating point. It means that the output of the engine drops over time. The engine may be able to produce a designed output at a certain rotational speed when it is new, but over time the engine will require an increased amount of fuel to produce the designed output at the same rotational speed. When a gas turbine engine operates at an operating point that provides a high efficiency and a low emission, the internal temperature of the engine is maintained at a level near a limit temperature. However, if an increased amount of fuel is supplied, the internal temperature may exceed the limit temperature, and it would adversely affect the various component parts. If such is a case, more frequent servicing would be required, and the maintenance cost of the engine would be increased.
Therefore, conventionally, during the operation of a gas turbine engine, a temperature sensor is used for detecting the internal temperature (either turbine inlet pressure TIT or turbine outlet pressure TET), and conduct a feedback control by using this temperature data and shifting the rotational speed to a higher range so as to achieve the rated output without increasing the detected internal temperature beyond the limit temperature. However, because the internal temperature may also be affected by the inlet state (such as the atmospheric temperature and atmospheric pressure), it is not possible to distinguish if a drop in the engine output is owing to the degradation of the engine or the change in the inlet state. Therefore, the engine may continue to be operated at a low efficiency although it is due to the degradation of the engine until it is determined by the next inspection of the engine.
Also, in a transient state of the engine such as when accelerating, because a feedback control may not be able provide a required quick response owing to the lack of responsiveness in the temperature sensor, it was proposed to use a schedule control (which determines the amount of fuel supply from a predetermined fuel map). However, as the degradation of the gas turbine engine progresses, the fuel map used for the schedule control becomes far from optimum so that it becomes necessary to set up the schedule with a sufficient margin and this prevents the engine to be operated in an optimum fashion.
To produce a designed output even after the degradation of the engine has progressed to a certain extent, it was proposed to compute the amount of fuel supply by multiplying a proportional gain to a difference between the target rotational speed and the actual rotational speed and adding to this value the amount of fuel supply for a zero-load condition as disclosed in Japanese patent laid open publication No. 2002-4889.
This enables the engine to produce a designed output even when the degradation of the engine has progressed to a certain extent, but it is inevitable that the gas turbine engine to be operated at a relatively low efficiency because the internal temperature of the engine is not taken into account.